A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was: is patient-prosthesis mismatch an independent risk factor for 30-day and mid-term overall mortality in adult patients undergoing aortic valve replacement (AVR)? Altogether, almost 400 papers were found using the reported search, of which 22 represented the best evidence to answer the clinical question. The authors, journal, date and country of publication, patient group studied, study type, relevant outcomes and results of these papers are tabulated. The majority of the selected articles have focused their analysis on moderate mismatch defined mostly by the presence of an indexed effective orifice area (IEOA)<or=0.85 cm(2)/m(2). In fact, because of the low incidence of patients with severe mismatch, these were often grouped for the statistical analysis with patients with moderate mismatch. Only six studies have analyzed the specific condition of severe mismatch (IEOA<0.65 cm(2)/m(2)). Three studies used the IEOA or indexed geometric orifice area (IGOA) as a continuous variable. We conclude that there is no strong evidence that moderate patient-prosthesis mismatch (PPM) (indexed IEOA<or=0.85 and >0.65 cm(2)/m(2)) is an independent risk factor for 30-day or mid-term overall mortality for adult patients undergoing AVR. An exception could be represented by patients with poor ejection fraction, a condition that can make moderate mismatch a predictor of overall mortality after AVR. On the other hand, severe mismatch is a predictor of overall 30-day or mid-term mortality for patients undergoing AVR independently from the presence of poor ejection fraction. In conclusion, our review suggests that the condition of severe PPM should be always avoided, while the presence of moderate mismatch could be tolerated in patients with normal ejection fraction without any impact on overall survival.